Matt Yglesias says that legal marijuana would be amazingly cheap:
One key but little-appreciated fact is that, according to persuasive research by Jonathan Caulkins, Angela Hawken, Beau Kilmer, and Mark Kleiman in their new book Marijuana Legalization: What Everyone Needs To Know, is that legal pot would be amazingly cheap. In fact, midgrade stuff would be so cheap that it might make sense for businesses to give it away like ketchup packets or bar nuts.
....This would make pot far and away the cheapest intoxicant on the market, absolutely blowing beer and liquor out of the water. Joints would be about as cheap as things that are often treated as free. Splenda packets, for example, cost 2 or 3 cents each when purchased in bulk.
Probably nobody cares about this, but there's a reason marijuana isn't legal anywhere in the world: the Single Convention on Narcotic Drugs, an international treaty adopted in 1961. Marijuana is a Schedule I drug under this treaty, which means it's flatly, totally forbidden. Countries can decriminalize marijuana use, but no signatory to the treaty can legalize either use or cultivation.
So it's not just a matter of getting either Congress or a state legislature on board for legalization. You'd have to get the United States to withdraw from the 1961 treaty, and that just isn't in the cards. Decriminalization and wink-wink-nudge-nudge lack of enforcement are about the best we can hope for anytime in the near future.